The fabrication of semiconductor devices often requires the careful synchronization and precisely measured delivery of as many as a dozen gases to a processing tool, such as a vacuum chamber. Various recipes are used in the fabrication process, and many discrete processing steps can be required, where for example a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, or metalized. The steps used, their particular sequence and the materials involved all contribute to the making of particular devices.
Accordingly, wafer fabrication facilities are commonly organized to include areas in which chemical vapor deposition, plasma deposition, plasma etching, sputtering and other similar gas manufacturing processes are carried out. The processing tools, be they chemical vapor deposition reactors, vacuum sputtering machines, plasma etchers or plasma enhanced chemical vapor deposition chambers, or any other device, apparatus or system, must be supplied with various process gases. Pure gases must be supplied to the tools in contaminant-free, precisely metered quantities.
In a typical wafer fabrication facility the gases are stored in tanks, which are connected via piping or conduit to a gas delivery system. The gas delivery system includes a gas box for delivering contaminant-free, precisely metered quantities of pure inert or reactant gases from the tanks of the fabrication facility to a process tool and/or chamber. The gas box typically includes a plurality of gas flow lines each having a flow metering unit, which in turn can include valves, pressure regulators and transducers, mass flow controllers, filters/purifiers and the like. Each gas line has its own inlet for connection to a separate source of gas, but all of the gas paths converge into a single outlet for connection to the process tool.
Sometimes dividing or splitting the combined process gases so that they can be delivered to multiple locations of a single tool or among multiple processing tools is desired. In such cases, the single outlet of the gas box is connected to the multiple locations through secondary flow lines. In some applications, where, for example, the upstream pressure needs to be kept lower than atmospheric pressure (e.g., kept at 15 PSIA) for safety or other reasons, a flow ratio controller is used to insure that the primary flow of the outlet of the gas box is divided in accordance with a preselected ratio among the secondary flow paths. Examples of split flow systems are described in U.S. Pat. Nos. 4,369,031; 5,453,124; 6,333,272; 6,418,954 and 6,766,260; published U.S. Application No. 2002/0038669 and the pending parent application U.S. application Ser. No. 11/111,646, filed Apr. 21, 2005 in the names of Junhua Ding, John A. Smith and Kaveh Zarkar, and assigned to the present assignee (Attorney's Docket 56231-526, MKS-158). The flow ratio controller of U.S. Pat. No. 6,766,260 is of particular interest because each secondary flow line is controlled with a separate flow sensor and control valve.
Flow ratio controllers of the type shown in U.S. Pat. No. 6,766,260 will stabilize to the desirable ratio split after being initially set, but flows take time to stabilize, and in some applications this can be unsatisfactory. Further, the pressure drop across the flow ratio controller is high, and the controller provides poor control performance for handling downstream blocking of one of the secondary flow paths. Additionally, the system can be difficult to set up because of difficulties in initially determining fixed valve positions of the valves in the secondary flow lines. And for current embodiments using two secondary flow lines it is necessary to assign the high flow valve as the fixed valve and the low flow valve as the controlled valve for flow ratio control.
All of these prior art flow ratio controllers are designed to control the relative ratio of only two secondary flow lines. The issues become even more complex when the relative ratios of more than two secondary flow lines of a distributed system are to be controlled. A linear time invariant (LTI) system using a state space approach would provide insufficient dynamic range, and the nonlinear valve curves of the control valves make it difficult, if not impossible to use only one set of linearized model coefficients to describe a multiple-channel flow ratio controller (hereinafter “MCMFC”). Further, LTI systems require control of select variables. Good candidates to use as state variables are pressures, temperatures and flow rates. However, such variables are not always observable. For example, there is no way for a thermal sensor to sense absolute flow rate. In addition, there is no pressure information available to use for estimating these state variables. Finally, a high order state space controller may not be suitable for distributed systems where flow ratio through three or more lines need to be controlled.